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    • 专利摘要:
    Method and system for filtering effects of atmospheric fading on optical communication signals in free space. The method comprises introducing a data signal into a transmitter (TX) by providing an optical signal through an atmospheric communication channel; receiving, by a plurality of optical aperture receivers (AP1, AP2 ... APL), the optical signal and adding to each optical signal a delay, wherein each of said plurality of optical aperture receivers (AP1, AP2. .. APL) is located on a different fiber optic line (L1, L2 ... LN), spatially separated, and in which said delay is different for each of said different optical lines (L1, L2 ... LN); combining, by means of a fiber combiner (CMB), the plurality of delayed optical signals providing an individual optical signal; acquiring, through an optical receiver (RX), the individual optical signal and digitizing the individual optical signal by means of an analog-to-digital converter; and processing and equalizing by means of a digital signal processor the individual optical signal acquired and digitized by implementing an equalization algorithm to recover said input data signal. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2657902A1
    申请号:ES201631479
    申请日:2016-11-18
    公开日:2018-03-07
    发明作者:Aniceto BELMONTE MOLINA
    申请人:Universitat Politecnica de Catalunya UPC；
    IPC主号:
    专利说明:

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    DESCRIPTION
    Method and system to filter atmospheric fading effects in optical communication signals in free space
    Technical field
    The present invention relates generally to the field of atmospheric transmission alterations. In particular, the invention relates to a method and a system for filtering atmospheric fading effects in coherent or incoherent free space communication signals such as laser signals.
    Background of the invention
    Coherent laser communications through the atmosphere are difficult because turbulence alters the received optical signals and their mixing with a local oscillator [1]. In atmospheric systems, when the beam with compromised coherence overlaps with the perfect local oscillator beam, the mismatch of the two fields affects the downward conversion power. Field conjugation arrays use adaptive combination techniques with multi-opening receivers to improve coherent link performance by mitigating the consequences of turbulence on the coherent down-conversion power.
    Several adaptive techniques are currently used to maximize the coherent downlink power and significantly reduce the results of the turbulence in the coherent downlink performance. Receiving systems that employ individual monolithic apertures use adaptive optical systems such as deformable mirrors for phase compensation and mitigate atmospheric turbulence [1] - [2].
    As an alternative, multiple aperture receivers combine signals detected by several apertures to facilitate deep fading and recover detection efficiency [3] - [4]. However, this motivates the use of complex receivers since the optical signals collected by different openings are subject to separate down conversion and the corresponding emitted signals have to undergo adaptive processing, match their phases and adjust to scale before combining. .
    Some known documents that provide solutions to reduce atmospheric fading are:
    US-A1-2016164602 discloses a coherent optical receiver device that includes a first unit configured to emit local oscillation light having a predetermined wavelength, a second unit configured to receive an optical signal that has been introduced causing the optical signal interferes with the local oscillation light, a third unit configured to electrically process the optical signal received by the second unit, a fourth unit configured to monitor at least a portion of the optical signal power, and a fifth unit configured to control the power of light
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    local oscillation emitted by the first unit depending on an optical power monitored by the fourth unit.
    Document US-B2-US8036541 discloses a coherent optical receiver that includes a 90 degree optical hybrid circuit in which a received signal light is introduced, channel I and channel Q photodetectors into which the outputs of the hybrid circuit, a clock extraction circuit that reproduces a clock whose speed is equal to the demodulated signal obtained by demodulating the received signal light and synchronizing with it, sampling channels of channel I and channel Q that sample the outputs of signal from the channel I and Q channel photodetectors through the use of the clock, and a digital signal processing section that digitally processes the sampled signals, converts them into a digital signal and emits the digital signal. The digital signal processing section feeds a phase deviation signal detected back into the clock extraction circuit in order to control the clock phase, and compensates for the light scattering within a fiber and the phase fluctuation during propagation in free space.
    Document US-B1-8995841 discloses a system and method in the field of optical communications in free space (FSOC) to overcome the variations of spatial optical signals induced by the atmosphere operating within each of two FSOC terminals which constitute a bidirectional FSOC link, each terminal providing the fast adaptive beam path method along a much wider field of view than normally used for adaptive optical techniques. Each terminal uses a real-time adaptive beam addressing technique that continuously measures optical power and optical power gradients through receiver optical detectors; This data is sent to a control system that responds automatically by aligning the optical system accordingly, maximizing the optical signal power measured by the optical power reception detector.
    US-A-5093563 discloses an optical imaging system that includes a matrix of small aperture subtelescopes each with heterodyne detectors. The matrix detects the amplitude and phase of light waves that arise from a scene that is being observed before combining to form an image. The combination and beam interference functions are performed after detection through the use of electronic signal processing. Large aperture resolution is synthesized by electronic detection and correction of phase errors without compensating the optical phase of components. Parallel processing and compensation of atmospheric turbulence are achieved.
    However, simple and more versatile receivers are needed to achieve atmospheric fading reduction in coherent and incoherent optical communications.
    Bibliography:
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    [1] H. G. Sandalidis, T. A. Tsiftsis, and G. K. Karagiannidis, “Optics! wireless Communications with heterodyne detection over turbulence channels with pointing errors, ”J. Lightwave Technol. 27, 4440-4445 (2009).
    [2] A. Belmonte and J. M. Kahn, "Performance of synchronous optical receivers using atmospheric compensation techniques," Opt. Express 16, 14151-14162 (2008).
    [3] AR Weeks, J. Xing, R. Phillips, LC Andrews, CM Stickley, G. Sellar, JS Stryjewski, JE Harvey, “Experimental Verification and Theory for an Eight-Element Multiple-Aperture Equal-Gain Coherent Laser Receiver for Laser Communications, ”Appl. Optics 37, 4782-4788 (1998).
    [4] A. Belmonte and J. M. Kahn, "Sequential Optimization of Adaptive Arrays in Coherent Laser Communications," J. Lightw. Technol 31, 1383-1387 (2013).
    [5] J. G. Proakis and M. Salehi, Digital Communications, (Mc Graw-Hill, 2007).
    [6] S. Qureshi, "Adaptive equalization," Proc. IEEE 73, 1349-1387 (1985).
    [7] G. D. Forney, “Maxim um likelihood sequence estimation of digital sequences in the presence of intersymbol interference,” IEEE Trans. Inf. Theory 18, 363-378 (1972).
    [8] G. D. Forney, "The viterbi algorithm," Proc. IEEE 61,268-278 (1973).
    Description of the invention
    Embodiments of the present invention provide, according to a first aspect, a method for filtering atmospheric fading effects in optical communication signals in free space, such as laser signals. The proposed method comprises introducing a data signal into a transmitter providing as a result an optical signal to be transmitted through an atmospheric communication channel; receiving, through a plurality of optical aperture receivers, the transmitted optical signal and adding a delay to each optical signal received by each of the plurality of optical aperture receivers; combining, through a fiber combiner, the signals received from each of the plurality of optical aperture receivers providing an individual optical signal; acquire, by means of an optical receiver, the individual optical signal provided and digitize the individual optical signal by means of an analog to digital converter; and process and equalize by means of a digital signal processor the individual digitized optical signal by implementing an equalization algorithm to recover said data signal introduced into the transmitter. Therefore, with the proposed method, the atmospheric communication channel is compensated in the digital domain.
    According to the invention, the optical communication signals in free space can be either coherent or incoherent. In the particular case that they are coherent optical communication signals, the fiber combiner combines the plurality of delayed optical signals with a frequency of electronic means such as a local oscillator, which is in connection with the optical receiver.
    According to the invention, each of the plurality of optical aperture receivers is located in a different fiber optic line, which are spatially separated and
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    preferably they are single-mode fiber lines. The different fiber optic lines can be arranged in an array of optical fiber. In addition, the included delay is different for each of the different optical lines.
    Preferably, each of the different optical lines of the fiber optic matrix has a different length. In this case, according to one embodiment, the delay is added by modifying the length of each of the different optical lines of the fiber optic matrix.
    The delay can alternatively be added using an optical device that includes a delay line that creates a time difference between each of the different optical lines.
    The delay is sequentially increased from one fiber optic line to another, in which the first fiber optic line preferably has the lowest delay and the last fiber optic line has the greatest delay, or vice versa.
    According to a preferred embodiment, the delay increase between fiber optic lines has a length of a period T of a symbol of the transmitted optical signal. Alternatively, according to another embodiment, the delay increase between fiber optic lines is half the length of a period T of a symbol of the transmitted optical signal. Other delays may also be possible without departing from the scope of protection of the present invention.
    Embodiments of the present invention provide, according to another aspect, a system for filtering effects of atmospheric fading into coherent or incoherent free space optical communication signals. The system is configured to implement the first aspect method.
    The present invention provides a new reception diversity scheme that converts spatial diversity into delayed signals and uses adaptive digital filters to obtain diversity gain from fading. The proposed diversity scheme allows the phase and amplitude compensation of the complex optical field and makes coherent or incoherent optical communications reliable in most practical propagation conditions.
    In addition, the proposed diversity scheme is extremely simple and solves the problem of adaptation in the digital domain. In addition, it provides a faster adaptation speed for the atmospheric communication channel, up to hundreds of kilohertz, estimating and applying control signals in the digital domain.
    Brief description of the drawings
    The above and other advantages and characteristics will be more fully understood from the following detailed description of embodiments, with reference to the attached figures, which should be considered in an illustrative and non-limiting manner, in which:
    Fig. 1 illustrates a system for filtering atmospheric fading effects in coherent optical communication signals according to an embodiment of the invention, in particular in coherent laser communications. In Fig. 1 (a), in laser communications
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    in free space, coherent diversity receivers implemented using a plurality of L optical aperture receivers are used to mitigate atmospheric fading. In Fig. 1 (b), the proposed system converts spatial diversity into delayed signals and overlaps the equivalent of L atmospheric coherence periods within the scope of a symbol. In Fig. 1 (c), the overlap in the receiver of multiple delayed copies of the transmitted signal creates interference between symbols (ISI) and an artificial multipath distortion. After the receiver, which recovers the phase and amplitude of the complex signal, an analog to digital A / D converter is used together with digital signal processing (DSP) manipulation (digital equalizer).
    Fig. 2 illustrates the symbol error rate (SER) for BPSK modulation as a function of the number of signal photons (SNR) per Q symbol collected in the matrix. (a) SER measurements compare the performance of LE, DFE and MLSE equalizers with an optical array of 13 elements. For comparison, the corresponding result is provided for the diversity of 13 openings receivers with maximum ratio combination (MRC). (b) SER performance is shown for different numbers of opening L receivers in the matrix when an MLSE equalizer is considered. In both graphs, the corresponding curves are shown without diversity (individual opening) and without fading (AWGN limit) for reference.
    Detailed description of the invention
    Fig. 1, which has been divided into three parts (a, b, c) to improve readability, illustrates an embodiment of the present invention for coherent free space laser communications. For the coherent receiver system in Fig. 1 (a), the atmospheric optical signal received by the different aperture receivers AP1, AP2 ... APL (see Fig. 1 (b)) shows field fading, i.e. random fluctuations of both the envelope and the phase over time. After the downward conversion of the optical signals, the fading will cause destructive interference in the combiner and reduce the total sign intensity.
    The fading in the plurality L of opening receivers AP1, AP2 ... APL can be added in a complex channel column vector a = (a1, a2, ..., aL) T e where the superscript T indicates transposition. A general input of the atmospheric fading vector is indicated by a, = | a; | is: pO'0í), where | a¿l represents the envelope with fading and <¡> the corresponding random phase of the optical signal to the opening ; and i'i 3 3}. For a coherent receiver limited by impact noise, the signal-to-noise ratio
    (SNR) composed of symbol Y = I can be taken as the number of signal photons received at the multiple receiver aperture I multiplied by a heterodyne mixing efficiency "2 = IEi" il2.
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    When considering a linear combiner together with the matrix, the received signal is the result of adding together the scaled and phase shifted signals received from the various opening receivers AP1, AP2 ... APL:
    r (t) = a, s (t) + n, (t)

    (one)
    where "(O is white Gaussian detector noise. It is assumed that the dominant noise source is the impact noise of the local oscillator laser, which can be accurately modeled as additive white Gaussian noise that is statistically independent of atmospheric fading.) transmission signal waveform s (t) = p (t-íiT) for the sequence of data symbols d {íi), ne {1,2, ..., JV} of
    Length N is the sum of pulses with the form p (t) transmitted by symbol interval T = l / B, when B is the signal spectral bandwidth. In this case, an adaptive linear combiner w = (w1, w2, ..., wL) T e <CL is considered to compensate for the effects of fading and to match the matrix consistent with the optical input field [4], The Complex weight ^ of the linear combiner applied to the ith sub-opening output can be largely characterized as = | w¡ | exp (/ 0i) where | w, | ye¡ are the amplitude and phase controls, respectively, provided by the linear combiner. It is recognized [4] that, if instantaneous atmospheric fading information is known for all opening receivers AP1, AP2 ... APL in ec. (1), a field conjugation matrix that combines w = 'i-Vf makes a perfect combination possible and produces perfect mixing of the matrix signals. Now, the composite SNR resulting from the envelope detector for a maximum ratio combiner of l elements is the sum of the SNR elements, that is, 7 = I EíI ^ I2 and a fraction equal to «¿l2 of the received photons incident y0 by symbol they are coupled to the i-th opening receiver.
    It is worth mentioning that the interference between symbols between two symbol transmissions is negligible in optical channels in free space due to the effects of multiple extremely small trajectories in the atmosphere. In addition, since the optical signal rates l / T increase to several GHz, atmospheric communication channels are best described as slow fading channels, in which the atmospheric channel remains constant over a coherence time t greater than the duration of transmission of the symbol T. Generally the rate 1 / t at which the atmospheric turbulence fluctuates is not greater than 1 kHz. In this regime of negligible interference between symbols and slow fading, the atmospheric channel is flat, that is, it affects all frequencies in the signal spectral bandwidth B = 1 / T equally.
    Fig. 1 summarizes the main features of the proposed system for coherent optical communications in free space. In this case, the overlap in the RX receiver of multiple delayed copies of the transmitted optical signal (by the TX transmitter in Fig. 1 (a)) creates an artificial multipath distortion: The system converts spatial diversity into delayed signals and overlaps, preferably within the scope of a
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    symbol T, the equivalent to L periods of atmospheric coherencer. The constructive and destructive superposition of incoming symbols results in interference between symbols and produces selective fading for the frequency, that is, it affects more at some frequencies along the spectral bandwidth of signal B than others.
    The system considers a plurality L of optical aperture receivers AP1, AP2 ... APL, which can be arranged in a fiber array, each optical aperture receiver AP1, AP2 ... APL feeding a single mode fiber line L1 , L2 ... LN (preferably of different lengths), and in which the fields of each optical aperture receiver AP1, AP2 ... APL are appropriately delayed, for example by controlling the length of each fiber optic line L1, L2 ... LN, and are added in a CMB fiber power combiner (Fig. 1 B)). The output is then available in an individual fiber and overlaps with a local oscillator (LO) field in a directional coupler. The RX receiver uses balanced detection and a digital sampler, connected directly to the detector's output port element, so that the electrical signal subjected to instantaneous downward conversion can be measured consistently (Fig. 1 C)). Although other devices can be used to delay the optical signals in the proposed system with symbol separation, the optical fibers are attractive delay lines due to their flexibility and low propagation losses, especially for high data rates that require a bandwidth B = l / T of up to several GHz. Each successive fiber optic line L1, L2 ... LN needs to have an additional length vr with respect to the previous one, v being the group speed of the light in the fiber. For a working wavelength of 1550 nm, where you see approximately 2/3 of the speed of light in a vacuum, and over the 0.1 ns time interval of a symbol at data rates of 10 GB, the relative additional length is only 2 cm.
    In the present invention, no adaptive linear combiner w is considered and the received signal r (t) is generated from the delay overlay with symbol separation T and the interference of the signals received from the plurality L opening receivers AP1, AP2 ... APL:
    r (t) = / a¡ s (t - l T) + n (t)
    2)
    It is well known that channel equalization is necessary at the RX receiver to mitigate the effect of interference between symbols [5] so that, if the channel is selective for the frequency in the spectral bandwidth of the physical signal, the equalizer Power frequency components with small amplitudes and attenuate those with larger amplitudes. The objective is that the combination of channel, symbol interference and digital equalizer filter provide a flat received frequency response and a linear phase in the signal spectral bandwidth B.
    The equalization procedure to mitigate the effects of interference involves using digital methods to collect the scattered symbol energy s (t-IT) in ec. (2) back to its original time interval s {t) so that it does not complicate the detection of others
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    Symbols Simultaneously, a digital equalizer also provides diversity by synthesizing a reverse filter of the lla * atmospheric channel and applying it to the different components of the collected scattered signal. Together, the equalizer seems to act in the temporal domain as does the perfect maximum ratio combiner in the spatial domain by digitally providing the RX receiver with symbol energy that would otherwise be lost by interference.
    There are benefits of using a coherent RX receiver when using additional digital signal processing (using the DSP digital signal processor of Fig. 1 (c)) for interference compensation. While the signal received after direct detection (square law) is proportional to the optical power received, in a coherent RX receiver the electrical signal received is proportional to the optical field. As a result, since atmospheric signal distortions can be expressed as linear transfer functions that act on the complex amplitude of the optical signal, in principle they can be compensated by linearly equalizing the detected complex amplitude consistent with digital techniques.
    A simple means of implementing such digital signal processing in the proposed system, implemented with a linear filter, is the use of an adaptive linear equalizer with symbol separation (LE) [6], in which the detection of the data sequence d (n) can be obtained by sampling the output signal at intervals synchronized with the symbols, that is,
    L
    y [n] = y (nT) = V cl r (n T - IT) -
    (3)
    In this case, a set of weighting coefficients c = (/: ■ _ ■■■: .. 'e ZL, describing the sockets of a finite pulse response filter is used
    linear (FIR), to compensate for interference between symbols and atmospheric fading effects on the received signal r (t). The number of FIR shots in ec. (3) is equal to the number of i opening receivers AP1, AP2 ... APL and the ith take implements a temporary IT delay. A natural extension of the LE is the decision feedback equalizer (DFE), a filter that depends on the idea that, once the value of the current transmitted symbol has been determined, the contribution to interference between symbols of that symbol in Future received symbols can be deleted accurately. DFE consists of an LE with an additional filter to process past symbol decisions d (n) in order to cancel any remaining interference between symbols.
    Among all digital equalization techniques, the maximum probability sequence estimation (MLSE) invokes the optimal receiver in terms of minimizing the error rate of the data sequence [7], Given a set of N observations and W, the function of probability X {d [n] | y [n]) is maximized by the most likely transmitted data sequence The Viterbi algorithm [8] can be used to recursively resolve to determine the optimal transmitted sequence d [n] consistent with the observations yM minimizing the cost function / = £ J r [n] - ZiQ tf [n -1] p [í - ít] | 2, where p (t) is the form of
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    Pulse transmitted at each symbol interval. Like all other equalization techniques, MLSE works well because c¡ coefficients are determined from training patterns affected by atmospheric alterations. The coefficients c = ■■ 'of the equalizer can be adapted to the characteristics of the atmospheric channel
    variables over time using a variety of methods such as the least squares algorithm (LMS).
    According to one embodiment, the present invention considers the performance of binary phase shift modulation (BPSK) using coherent detection in the presence of atmospheric fading and additive white Gaussian noise (AWGN), in a non-limiting manner since any complex modulation can also be used such as phase shift (M-PSK) in which a finite number M of phases is used to modulate the signal, frequency shift (M-FSK), amplitude shift (M-ASK), or quadrature amplitude modulation (M-QAM), among others. The performance offered by the proposed system is illustrated when the three main types of current equalizers (i.e., LE, DFE, and MLSEE) are implemented in the RX digital receiver and face typical atmospheric channel conditions. A numerical simulation analysis has also been performed and a set of experiments with synthetically constructed network signals has been performed. They are considered receivers of the openings AP1, AP2 ... APL and it is assumed that each opening receiver couples the received optical signal (light) in a single mode fiber L1, L2 ... LN.
    Fig. 2 considers the symbol error rate (SER) as a function of the total number of signal photons and 0 collected on each fiber optic line L1, L2 ... LN when digital equalization techniques are used. In both graphs, the corresponding curves are shown without diversity (individual opening) and without fading (AWGN limit) for reference.
    Fig. 2 (a) compares the performance of the three different equalizers described above (in a non-limiting manner since other equalizers can also be used) when used together with a matrix of 13 elements (13 AP aperture receivers). It is evident from the graphs that the LE filter does not work well and, in the best case, can only offer a simple gain of diversity of 3 dB against the strong atmospheric fading considered in this case. As expected, the DFE filter works much better and achieves a diversity gain greater than 10 dB at a symbol error rate of. Finally, the most complex MLSE equalizer shows almost optimal performance. Sequences of data with a length JV = 2048 symbols have been considered. For comparison, the corresponding results are provided for a receiver of a 13 branch field conjugation matrix. It shows how multi-aperture receiver systems with signals received with optical delay and MLSE equalization provide a diversity gain within a fraction of dB of that with complex reception field conjugation networks. In the strong atmospheric conditions considered in these graphs, a receiver of 13 elements of moderate size, which is easily managed by means of
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    MLSE filter, produces a great diversity gain of 14 dB at a SER of 10-3. It is indicated that the MLSE filter requires a Viterbi algorithm and the complexity increases exponentially with the number of interference symbols and L jacks. For a BPSK modulation, in which the size of the symbol letter is 2, the Viterbi algorithm Calculate 2i + 1 metrics for each new symbol received. As a consequence, the practical use of the MLSE equalizer is limited to an array of receivers with a non-excessive number of openings l. On the other hand, DFE has a computational complexity that grows linearly with the number of L sockets and may be more convenient when large aperture matrices are needed.
    In Fig. 2 (b), an MLSE equalizer is considered for different numbers of AP optical aperture receivers. It can be seen that it requires a SNR of
    -3
    approximately 7 dB (5 photons) per symbol to maintain a SER of 10 “in AWGN, while requiring an SNR of more than 22 dB (158 photons) per symbol to maintain the same error rate in atmospheric fading (individual opening) . Simulated symbol error rates are provided for systems with 3, 7 and 13 opening receivers. For equalization of the delayed signals collected through only three aperture receivers, the system requires a 12 dB SNR to maintain the SER of 10 “3. This represents a 5 dB SNR fade penalty with respect to the AWGN limit. For receivers with 7 openings and 13 openings, the SNR penalty is reduced to 3 dB and 1 dB, respectively.
    Although in the previous explanations it has been considered that the delay increase between fiber optic lines L1, L2 ... LN has a length of a period T, other delays can also be applied for each AP optical aperture receiver. For example, the delay increase from one fiber optic line to another can be half the length of a period r, among others.
    In addition, the present invention can also be used to filter atmospheric fading effects in optical communications inconsistent in free space. In this particular case, and differently from what is illustrated in Fig. 1, no electronic means such as the local oscilloscope LO are used, with the output of the CMB fiber combiner directly connected to the input of the RX optical receiver, in this case an incoherent optical receiver.
    Furthermore, although in the embodiment of Fig. 1 the analog and digital A / D converter and the DSP digital signal processor are independent units of the RX optical receiver, in alternative embodiments of the invention, in this case not illustrated, the receiver itself RX optical can include an integrated circuit, such as a field programmable gate network (FPGA), which implements an analog and digital A / D converter and a DSP digital signal processor.
    Referring now to Fig. 3, there is shown an embodiment of a method for filtering atmospheric fading effects in optical communication signals in space
    free, either coherent or incoherent. According to the proposed method, in step 301, a plurality of optical aperture receivers AP1, AP2 ... APL receive an optical signal transmitted by a TX transmitter through an atmospheric communication channel. Each optical aperture receiver AP1, AP2 ... APL is located on a spatially separated optical line L1, L2 ... LN, 5. Then, in step 302, each optical aperture receiver AP1, AP2 ... APL adds a delay to its received optical signal. Next, in step 303, a CMB fiber combiner combines the signals received from each optical aperture receiver AP1, AP2 ... APL (delayed signals) to give an individual optical signal. Next, an RX optical receiver acquires the individual optical signal and digitizes it (step 305) by means of an analog to digital converter. Finally, in step 306, the digitized signal is processed and equalized by a digital signal processor that implements equalization algorithms.
    In the proposed method, an optical device (not illustrated) such as a delay line can be used in connection with each of the optical aperture receivers AP1, AP2 ... APL to add the delay. Alternatively, the delay can be included by modifying the length of each of the different optical lines L1, L2 ... LN.
    The above description is considered to be only that of preferred embodiments. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are for illustrative purposes only and are not intended to limit the scope of the invention, which is defined by the following claims.
    权利要求:
    Claims (16)
    [1]
    1. A method for filtering atmospheric fading effects in optical communication signals in free space, the method comprising:
    - insert a data signal into a transmitter (TX) providing an optical signal
    5 to be transmitted through an atmospheric communication channel;
    - receive, through a plurality of optical aperture receivers (AP1, AP2 ... APL), the transmitted optical signal and add to each optical signal received by each of said plurality of optical aperture receivers (AP1, AP2 ... APL) a delay providing a plurality of delayed optical signals,
    10 wherein each of said plurality of optical aperture receivers (AP1,
    AP2 ... APL) is located on a different fiber optic line (L1, L2 ... LN), spatially separated, and in which said delay is different for each of said different optical lines (L1, L2 .. .LN);
    - combine, by means of a fiber combiner (CMB), the plurality of optical signals
    15 delayed providing an individual optical signal;
    - acquire, by means of an optical receiver (RX), the individual optical signal and digitize the individual optical signal by means of an analog to digital converter providing a digitized optical signal; Y
    - process and equalize the optical signal using a digital signal processor
    20 digitized by implementing an equalization algorithm to recover said signal from
    data entered into said transmitter (TX).
    [2]
    2. A method according to claim 1, wherein the optical communication signals in free space are coherent optical communication signals, and wherein the plurality of delayed optical signals are combined by the fiber combiner (CMB) with
    25 a frequency of electronic means, including a local oscillator (LO), in
    connection with the optical receiver (RX).
    [3]
    3. The method of claim 1, wherein the optical communication signals in free space are inconsistent optical communication signals.
    [3]
    3. Method according to any of the preceding claims, wherein each of
    30 said different optical lines (L1, L2 ... LN) of the fiber optic matrix has a
    different length
    [4]
    4. The method according to claim 3, wherein said delay is added by modifying the length of each of the different optical lines (L1, L2 ... LN).
    [5]
    5. Method according to any of the preceding claims 1 to 3, wherein said
    Delay is added using an optical device that includes a delay line that creates
    a time difference between each of the different optical lines (L1, L2 ... LN).
    [6]
    Method according to any of the preceding claims, wherein said delay is sequentially increased from one fiber optic line to another, the first line having
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    fiber optic (L1) the shortest delay and having the last fiber optic line (LN) the longest delay.
    [7]
    A method according to claim 6, wherein said delay increase between fiber optic lines has a length of a period T of a symbol of the transmitted optical signal.
    [8]
    A method according to claim 6, wherein said delay increase between fiber optic lines is half the length of a period T of a symbol of the transmitted optical signal.
    [9]
    9. Method according to the preceding claims, wherein the different fiber optic lines (L1, L2 ... LN) are single mode fibers.
    [10]
    10. System for filtering atmospheric fading effects in optical communication signals in free space, comprising:
    - a transmitter (TX) into which a data signal is input and configured to provide an optical signal to be transmitted through an atmospheric communication channel;
    - different fiber optic lines (L1, L2 ... LN), spatially separated, each including an optical aperture receiver (AP) configured to receive the transmitted optical signal and to add a delay to its received optical signal, said being different delay for each of said different optical lines (L1, L2 ... LN);
    - a fiber combiner (CMB) in connection with each fiber optic line (L1, L2 ... LN) and configured to combine the signals received from each of the plurality of optical aperture receivers (AP1, AP2 ... APL) to provide an individual optical signal; Y
    - an optical receiver (RX) configured to acquire said individual optical signal, said individual optical signal being digitized by an analog to digital (A / D) converter and processed and equalized by a digital signal processor that implements an equalization algorithm to recover said data signal introduced into said transmitter (TX).
    [11]
    A system according to claim 10, wherein the optical communication signals in free space are coherent optical communication signals, and the system further comprises electronic means, including a local oscillator (LO), in connection with the optical receiver (RX ).
    [12]
    12. System according to claim 10, wherein the optical communication signals in free space are incoherent optical communication signals.
    [13]
    13. System according to claim 10, wherein each of the different optical lines (L1, L2 ... LN) has a different length.
    [14]
    14. System according to any of claims 10 to 13, further comprising an optical device, which includes a delay line, in connection with each of the
    Optical aperture receivers (AP1, AP2 ... APL) and configured to include a time difference between each of the different optical lines (L1, L2 ... LN).
    [15]
    15. System according to any of the preceding claims 10 to 14, wherein
    Different fiber optic lines (L1, L2 ... LN) are single mode fibers.
    5
    image 1
    Optical wavefront
    5
    image2
    image3
    <c)
    Fig. 1
    image4
    Fig 2
    image5
    0 2 4 6 8 10 12 14 16 18 20
    SNR (Pholocounts) per Symbol [dB]
    image6
    Fig. 3
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